Hybrid battery packs

Chat about all things battery in here.
Need to know what type to use or size or capacity then again place your thoughts here
MalcolmB
Posts: 423
Joined: Sat Jun 23, 2007 8:07 pm

Hybrid battery packs

Postby MalcolmB » Sun Jul 20, 2008 3:43 pm

In the latest issue of Plugged In (thanks Kelvin!) I was interested to see Frank Wykes’ article about hybrid battery packs. He talks about combining two different cell chemistries in order to take advantage of their respective strengths, e.g. high power/high capacity.

I’ve been thinking about this type of arrangement for a while for a lightweight car conversion, mainly to keep costs down. I don’t like the idea of weighing down a vehicle with a load of lead, but at the same time can’t afford a decent-sized lithium ion pack, so I’m considering a compromise until the price of lithium cells comes within reach. I’d appreciate any feedback on possible pitfalls of this approach. The idea is to build a small 108V pack as follows. Maximum discharge current would be around 300A.

Nine 12V Hawker Genesis lead acid 42AH batteries (ex backup power supply: £180 from eBay). Weight: 135 kg

108 x 3V LifeBatt LiFePO4 10 Ah cells (at around £20 per cell = £2160). Weight: 41 kg

The LifeBatt cells would be arranged in parallel groups of three cells (so 30 Ah), which would then be wired in series to give a nominal 108V.

The idea is to connect the lead acid string in parallel with the lithium ion string to give a nominal 108V 72 Ah pack. Because of the high discharge rate I would only expect to get about 25 Ah from the lead acid batteries, so the total available capacity would be roughly 50 Ah at 108V, or 5.4 kWh. At an estimated energy consumption of 250 Wh/mile this would give around 20 miles range.

The attraction of using this combination of cells/batteries is that their voltages match up neatly. The voltage of four LiFePO4 cells closely matches the voltage of a lead acid 12V monoblock (made up of six 2V cells). In addition, lead acid and lithium ion both use a similar constant current/constant voltage (CCCV) charging characteristic.

To test how the cells work together I’ve done a few measurements on a 36V 10Ah LifeBatt pack connected in parallel with a 36V 42 Ah Hawker Genesis pack. The parallel packs were charged from roughly 80% state of charge using a Curtis 36V/30A high-frequency battery charger adjusted to give a finishing voltage of 43.8V with the current tapering to about 4A. This is not the optimum charging characteristic for the lead acid batteries, but my main priority was to charge the more expensive lithium ion cells correctly. Here are some figures for comparison:

Final charging voltage:
Pb 43.8V (14.6V per battery)
Li 43.8V (3.65V per cell)

Voltage fresh off charger (after disconnecting the two strings):
Pb 40V (13.33V per battery)
Li 42.0V (3.5V per cell)

Resting voltage after 8 hours (strings disconnected immediately after charging)
Pb 38.5V (12.83V per battery)
Li 39.8V (3.32V per cell)

After measuring the resting voltages above I connected the lead and lithium strings back in parallel just to see how they got on together. Because the lithium pack has a slightly higher voltage it naturally feeds the lead pack. The initial current from the lithium string was about two amps. This tapered off quite quickly, reaching 0.12A after one hour, and 0.07A after three hours, so it seems that the strings could be left connected in parallel after charging without any real problem.

According to the Hawker spec sheet, at 20% state of charge (the lowest I would want to go), the voltage of the lead acid battery drops to just over 11.8V, which corresponds to 2.95V for the lithium cells. This seems a little high as a finishing voltage for lithium, but I guess it’s better to be on the safe side.

So far so good. Next I’d like to do a discharge test with the two strings connected in parallel, but I need to find a suitable load first. I’ve still got more questions than answers: How will the two strings share the load? What would be the optimum balance between lead and lithium string capacities? What effect will the greater sag of the lead acid batteries have on current sharing? Frank Wykes said in his brief article that “in no circumstances should high and low power sets of cells be operated in parallel, even if the voltage is the sameâ€Â

jonathan jewkes
Posts: 67
Joined: Sun Jul 15, 2007 8:14 pm
Location: Bristol

Postby jonathan jewkes » Tue Jul 22, 2008 10:55 pm

Very interested in your experiment. I have been thinking of a similar arrangement for some time, but never got around to actually doing anything, except for having brief chats with a couple of other BVS members. Frank's recent comment in Plugged-In put a damper on the idea - I'm sorry that he didn't give reasons for saying that you cannot connect dissimilar strings in parallel.
One suggestion is to put a diode (a heavy-duty one) between the Lithium and the Lead strings, with the charger connected to the Lead. Then both strings will charge up, but when you stop charging the Lithium cannot discharge into the Lead.
You connect the load (ie motor controller) to the Lithium string. Although the resting voltage of the leads will be less than the lithiums, the idea is that when load is applied , the voltage of the lithiums will drop more than the leads because of higher internal resistance, thus allowing both strings to contribute to the load.
This all depends on whether they do actually discharge in this manner - so I hope you can find a suitable test load to continue your experiment.
Good luck - and let us know the results.
Jonathan
Jonathan Jewkes
Daily EV user for 10years - an enthusiast and also a realist

uberdeity
Posts: 1
Joined: Fri Jul 25, 2008 1:49 pm

How about...

Postby uberdeity » Fri Jul 25, 2008 2:05 pm

...combining a high capacity pack with a low-capacity high discharge rate battery pack in a slightly different way?

Have some sort of switching system (possibly even just a switch!) that would allow you to change from high-current to high-capacity and back again. Have the high-current batteries feed the motor whenever you're accellerating, and have the high-capacity batteries feed the motor when you're going at a constant speed. They could also recharge the high-current batteries at the same time- removing some of the problems of entirely separate or just-connected-in-parallel battery packs. It also means that the rapid discharging is only happenning from the relatively cheap lead acid battery rather than the pricey lithiums- so any damage you do to the battery won't be expensive to fix.

An alternative to a dashboard-mounted switch could be a current-triggered relay that would kick in the lead acids whenever the current draw exceeds a certain pre-set level.

Either way you'd essentially end up with a relatively low-cost, high capacity, high peak power rating battery pack.


Return to “All things battery related”

Who is online

Users browsing this forum: No registered users and 33 guests